If you can still see the coloured bar across the top of the note,
right-click it and select Size. If you can't, Alt-Space should bring up the
same menu, then press S. Either way, your cursor will change to a
four-headed arrow. Press one of the arrow keys. This may make the cursor
jump off the screen if that's where the edge of the note is but don't worry.
First press of the arrow key snaps the cursor to the edge, then if you press
the opposite direction it'll drag the edge in.
For example:
Alt-Space (or right-click the Title bar)
S
Right arrow
Left arrow repeatedly until the edge comes into sight.
Enter (or right-click the Title bar)
Alt-Space
S
Up arrow
Down arrow repeatedly until the edge is in view
Enter.
Repeat with the other two edges if necessary. You should now be able to
resize with the mouse as normal.
If none of this is working, it might just be that you've maximized the
window, in which case just double-click the coloured bar at the top of the
note to restore it. In fact you may have to maximize and restore before
Alt-Space brings up the right menu. I've just tried it and sometimes it
brings up the File menu instead.
